Dispersions of particles of an inorganic oxide such as silica, alumina, zinc oxide, tin oxide, zirconia, and titania are employed in various industrial fields, particularly in the field of optics for modulating refractive index. Among those inorganic oxide particles, titania particles are employed preferably for increasing the refractive index of an optical material because they have a high refractive index.
As such a dispersion of inorganic oxide particles, an aqueous dispersion whose dispersion medium is water has been conventionally used. However, most of the optical material applications such as optical film production utilize such an aqueous dispersion usually as being mixed with resin components, and such an aqueous dispersion is kneaded only with difficulty in particular with a water-insoluble resin component. Accordingly, an organic dispersion whose dispersion medium is an organic solvent is becoming highly demanded in these days.
The inorganic oxide particles including titanium oxide particles are generally dispersible satisfactorily in aqueous solvents, but are poorly dispersible generally in organic solvents.
Such being the case, in the production of organic solvent dispersion of inorganic oxide particles, it is already known that surface treating the inorganic oxide particles with a silane coupling agent is effective in order to modify the inorganic oxide fine particles to be lipophilic (See, for example, Patent Literatures 1 and 2).
Also in the production of an organic solvent dispersion of titanium oxide, a method using a silane coupling agent has been proposed as described above. For example, there has been proposed a method in which a silane coupling agent is mixed with an alcohol dispersion of titanium oxide fine particles in the presence of acetic acid and the resulting mixture is stirred to surface-treat the titanium oxide fine particles, and then the dispersion medium of the alcohol dispersion of the titanium oxide fine particles is replaced by a lipophilic organic solvent such as methyl ethyl ketone, thereby obtaining a lipophilic organic solvent dispersion of titanium oxide fine particles (see Patent Literature 3).
However, the above-mentioned conventionally known methods are not successful in providing an organic solvent dispersion having a sufficient transparency depending on applications. In addition, the organic solvent dispersion obtained suffers poor stability such as gradual increase in viscosity. Such being the case, a higher performance of an optical material is becoming highly demanded in these days, and an organic solvent dispersion of titanium oxide particles which has a low viscosity and is superior in stability and transparency is highly demanded.